In vehicles with automatic transmissions, a torque converter or other type of fluidic coupling transfers torque from the vehicle engine (or other drive unit) to the transmission. Rather than requiring the vehicle operator to engage in manual gear-shifting, automatic transmissions have an on-board control system that can automatically control changes in the gear ratio once the vehicle is shifted into a forward or reverse range by the operator.
The on-board control system includes electro-hydraulic components. The hydraulic components include valving and fluid passages that direct pressurized hydraulic fluid to clutches to be applied and drain hydraulic fluid from clutches to be released, in response to electrical signals received from an electronic control unit.
The electro-hydraulic controls often include directional control valves, also known as shift valves or logic valves. The shift valves have ports that are coupled to fluid chambers of the transmission clutches. When the shift valves change position, ports on the valves are opened or closed as needed to direct pressurized hydraulic fluid to the appropriate clutches or other destination.
A solenoid often controls the changes in position of a shift valve. The solenoid receives electrical signals from the electronic control unit and outputs fluid pressure to the shift valve in response to the electrical signals. The fluid pressure output by the solenoid strokes the shift valve if the amount of pressure is sufficient to overcome the biasing force of the shift valve's return spring.
When the position of the shift valve changes, a clutch's fluid chamber may be connected to, or disconnected from, a source of pressurized hydraulic fluid. If the position of the shift valve connects a clutch to fluid pressure, the clutch is applied. The clutch is released if the position of the shift valve causes the clutch to be disconnected from the source of fluid pressure.
The electro-hydraulic controls also typically include pressure control valves, which may be known as trim valves. A trim valve controls the rate at which fluid pressure is applied to the clutch to which the trim valve is coupled (e.g. directly or via a shift valve). The trim valve thus controls how slowly or quickly the clutch connected to the trim valve is applied or released.